The present invention relates to a method for use in determining the shape of a flat fabric pattern that will be used to cover a general curved surface.
More particularly, the method relates to shaping the perimeter of a flat fabric prior to placement of the fabric on a foam core used in manufacturing composite parts by the resin transfer molding process. It should be well recognized that the method of the present invention may be applied to determine the required shape to cut an originally flat section of fabric to cover any (curved) surface.
As applied to the resin transfer molding process, the fabric will typically consist of random or oriented fiber bundles, wherein the fibers typically comprise fiberglass filaments.
In general, it is not possible to cover free-form curved surfaces with flat fabric without wrinkling or distortion of the original parallel configuration of the fiber bundles which form the fabric. Wrinkling and excessive distortion of the fabric are not acceptable since they yield a part susceptible to undesirable stress concentrations. A method therefore needs to be developed that will predetermine the pattern of a flat fabric used to cover a curved surface, wherein fiber spacing and fabric fiber orientation remain within specified tolerances after the fabric is laid upon the curved surface.
In the paper entitled "Fiber Placement During the Forming of Continuous Fiber Reinforced Thermoplastics" authored by Anthony J. Smiley and R. Byron Pipes, presented in the Composites in Manufacturing Conference Jan. 19-22, 1987 in Anaheim, Calif. and available from the Society of Manufacturing Engineers, a computational procedure which predicts the "as formed" fiber distribution of a single ply of unidirectional fibers is presented. Unfortunately, such a discussion deals only with one fabric layer at a time, and is restricted to surfaces that are symmetric, whereas the application of fiberglass cloth to a general non-symmetrical surface involves the interaction of a minimum of two layers of fiberglass fibers stitched to one another. This publication also models its mathematical assumptions on the premise that the distance between nodes on a single fiber is held fixed, whereas when multilayered fiberglass cloth is used the separation distance between adjacent fibers changes as the fabric is draped across the curved surface.
A method therefore needs to be developed wherein a multilayer fabric may be precut to a specific shape and then draped over a curved surface without the occurrence of adverse fiber distortion or wrinkling. The perimeter of the draped fabric should also match the boundary of the surface after the fabric has been oriented in a desired direction across the surface.